Electron multipliers



May 27, 1958 A. H. soMMER 2,836,755

ELECTRON' MULTIPLIERS Filed May 11. 1953 /7 www# /s l /nvfn72r- ALFREDHERMANN SUMMER United States Patent() ELECTRON MULTIPLIERS AlfredHermann Sommer, Princeton, N. J., assigner to Electric & MusicalIndustries Limited, Hayes, England, a British company Y Y ApplicationMay 11, 1953, Serial No. 354,112

r.Claims priority, application Great Britain May 27, 1952r 6 Claims.(CLSB-95) A This invention relates to electron multipliers of the typewherein one or more secondary electron emissive electrodes or dynodesare disposed within an envelope in which is'also disposed aphoto-electric cathode and an electron collecting electrode, thearrangement being such `that the incidence of light ontherphoto-cathode` causes `electrons to be emitted therefrom and theimpact of said electrons `on the secondary emissive electrode orelectrodes causes secondary electrons to be emitted in greater numbersthan the primary electrons whereby the electron current arriving at thecollector is greater than the electron current proceeding `from `thecathode.

For optical reasons it is desirable that the light intercepted by thephoto-cathode should` beincident in a direction normal to the surfaceVof the cathode and for electron-optical reasons it is also desirablethat the dynode `electrodes should be so arranged `as to ensure auniform transit time of the electrons and a uniform` collectionefliciency. t q

In a known form .of high sensitivity multiplier phototube theserequirements are achieved by employing a semitransparent photo-cathodeformed on the planar glass end wall of the envelope of theV tube, and aseries of venetian blind type of dynodes are provided so that they aresubstantially parallel to thesurface of the cathode. However, thephoto-electric sensitivity that can be obtained with semi-transparentcathodes is less than that which is obtainable with opaque `cathodes andhence, the usefulness `of the tube, particularly with regard tothersignalto-noise ratio could be greatly enhanced if a tube of thistype could be provided with an opaque photo-cathode.

It is therefore an object of the invention to provide an electronmultiplier device with an opaque photo-cathode' arranged so that lightmay be directed on to the cathode surface normally and wherein theelectron transit time from the cathode to the first` dynode issubstantially uniform. l l

According to the invention an Velectron multiplier comprising anenvelope having `a translucent window, an apertured dynode electrodemounted in said envelope to face said window, an electron emitting layeron the surface of said dynode remote from saidwindow, a furtherelectrode spaced from said dynode, and an opaque photoelectric cathode,means mounting said cathode between said dynode and further electrodewith the activesurface of said cathode facing said aperture in saiddynode,said cathode having an area small in relation to said dynode vandsaid further electrode, to allow electrons released from said dynode topass said cathode to said further electrode. Y l v Said furtherelectrodermaycomprise,a dynode or a collecting electrode' depending, onwhether the multiplier isa multi-stage or single stage device'.

In order to ensure that the secondary electronsemitted by said dynode,are directed onto said further electrode past said cathode focussingmeans may be provided and said focussing means may be in the form of aring surrounding the region between the cathode and further electrodeand maintained at or near the potential of the apertured dynode. Y

In order that the said invention maybe clearly underice stood andreadily carried into elect, various embodiments will now be describedwith reference to the accompanying drawings, in which:` v v Figure lshows one `embodiment of the inventionv applied to a multi-stageelectron multiplier, and

v Figures 2 and 3 show alternative forms of cathode which may beemployed. V s Y Referring to Figure 1 a tubular glass envelope 1 isprovided at one of its `ends with a planar transparent glass window 2and at its other end with a base 3 having sealed therethrough aplurality of contact makingelements 4 to which leads not shown, aretaken from the various electrodes. Arranged within the. envelope 1 near`to andlparallel with the windowV 2 is a metal discv which forms the rstdynode 5 of the device Vsaid dynode Shaving an active electron emittingVsurface 5a. This dynode 5 is provided with an aperture 6 so arrangedthat its centre in line with the axis of the envelope 1. The dynode 5may, for example be 2O mm. in diameter and the aperture 6,3 mm; indiameter, the surfaceA of the dynode 5 which is remote from the window 2being coated with Van active material such asY antimony caesium. Aphoto-cathode] in the form of a metal ,disc of 5 mm. diameter is mountedthe dynode 5 with its axis in line with the centre of the aperture 6 inthe dynode 5; the active surfaces of the two electrodes Sand 7 beingarranged facing each other but spaced apart by a distance of 15 mm. Thedistance between the cathode `7 and dynode electrode V5 has been foundto be fairly critical and should be of the order of 3 times the'diameter of the cathode 7.

A series of secondary electron emitting dynodes 8 and an electroncollecting electrode 9V are arranged between the cathode 7 and the base3, the first of this series of dynodes 8 being mounted so as to beat adistance of 5 mm. fromthe cathode 7 so that it faces the uncoated sidethereof. u q Each of the dynodes S comprises a box like structure 8ohaving arranged therein anumber of obliquely disposed louvres 10 theside of the structure 8a facing the cathode 7 being covered by wire mesh11. l

Extending from each structure 8a is a ange 12 provided with apertureswhereby the whole series of-dynodes 8 may be mounted by means ofinsulating rods 13 with the louvres 10 of adjacent dynodes 8 oppositelydirected as shown. In the drawing six dynodes 8 are illustrated by wayof example although it will be understood that the number provided maybe varied for different tubes depending on the degree of amplificationrequired. The collector electrode- 9V isisupported fromV an arm 14sealed through the wall of the envelope 1. Y

In the region between the vcathode 7 and the lirst dynode-8 there isarranged a tubular electrodelS of a diameter of l5 mm. `provided at theend nearest to the cathode with an outwardly directedange 16, saidelectrode being positioned so asV to be coaxial with the cathode 7 andserving in the operation of the tube asa means for focussing electronsfrom the apertured dynode 5 past the cathode 7 on to the rst dynode 8and thence through the further dynodes 8 Vto the collector`9. -The innersurface ofthe window 2` maybe provided as shown at 17 with a translucentconducting coating having `a con Y tact 1S passing through the envelope1, the purpose Yof this coating 17 will be described later.

The photo-cathode 7 and the dynodes 5`and 8 are 'pref- Y In operation ofthe device the apertured dynode is preferably maintained at 160 voltspositive and the first of the series of dynodes 8 at 480 volts positivewith respect to the cathode 7 with the focussing cylinder 15 at or nearthe same potential as the apertured dynode S, increasing positivepotentials being applied to the further dynodes 8 so that each dynode is160 volts positive with respect to the` preceding one the collectingelectrode 9 being 160 volts positive with respect to the lastV dynode 8of the series.

Light reaching the cathode 7 by passing through the aperture 6 in thedynode 5 releases photo-electrons which are attracted to the activesurface 5a of the dynode 5 to cause secondary electrons to be releasedtherefrom. It has been found that by suitably choosing the diameter othe cathode 7 and the spacing between the cathode 7 and dynode 5electrostatic eld conditions can be set up so that very few electronsemitted by the cathode 7 proceed to the area dening the aperture 6 inthe dynode 5, said electrons being substantially directed on to theactive surface thereof. Secondary electrons released from the dynode 5will .travel in the direction -of the more positive dynode 8 and due tothe focussing eld set up `by the tubular electrode and cathode 7 thesesecondary electrons will be directed past the cathode 7 so as to impingeon the first dynode 8 of the series thereby giving rise to furthersecondary electrons which are amplified in number bythe succeedingdynodes 8 in their passage to the collecting electrode 9.

Instead of making the cathode 7 of planar disc form, lcathodes havingconvex or concave active surfaces may be employed. In the case of aconvex cathode 7 illustrated in Figure 2 the photo-electrons releasedtherefrom are less likely to pass through the aperture in the dynode 5although there may be a slight tendency for such electrons to traveldirectly to the further electrode adjacent the cathode e. g. dynode 8.With a cathode 7 of concave shape as illustrated in Figure 3 directcollection of the photo-electrons by the further electrode e. g. dynode8 is not possible and provided it is arranged that a cross-over pointexists between the cathode 7 and the apertured dynode 5, loss ofphoto-electrons by passage through the aperture 6 is also reduced to avery low level.

As previously stated the inner surface of the window 2 in the region ofthe apertured dynode 5 may be coated with a translucent conducting layer17 and this layer is preferably, maintained at or near cathode potentialby means of a connecting strap 19, so as to set up an electrostatic eldin this region which has a repelling action i on the photo-electrons sothat substantially none of them are able to pass through the aperture 6.Such a conducing layer 17 may be formed by applying to the surface ofthe window 2 a solution of stannous chloride whilst the glass ismaintained at a high temperature.

Although the focussing cylinder 15 has been described as beingpositioned between the cathode 7 and lirst dynode 8, this electrode Vmaybe arranged at other suitable positions between the apertured dynode 5and the first dynode 8 so as to provide means for focussing thesecondary Velectrons past the cathode 7.

It will be understood that in the case of a single stage multiplier asuitable collecting electrode would be provided instead of the series ofdynodes 8 and such an electrode would be positioned so as to'face theuncoated side of the cathode 7.

What I claim is:

1. An electron multiplier comprising an envelope having a translucentwindow, an apertured dynode electrode mounted in said envelope to facesaid window, an electron emitting layer on the surface of said dynoderemote rom said window, a further electrode spaced from said dynode, andan opaque photo-electric cathode, means mounting said cathode betweensaid dynode and further -electrode with the active surface of saidcathode facing said aperture in said dynode, said cathode having an areasmall in relation to said dynode and said further electrode so as toprovide an open space surrounding said cathode, to allow electronsreleased from said dynode to pass said cathode to said furtherelectrode, and electrostatic focussing means arranged co-axially withthe centre of said cathode so as to lie between said dynode and saidfurther electrode but spaced from said cathode to direct electrons fromsaid dynode past said cathode onto said further electrode.

2. An electron multiplier comprising an envelope having a translucentwindow, an apertured dynode electrode mounted in said envelope to facesaid window, an electron emitting layer on the surface of said dynoderemote from said window. a further electrode spaced from said dynode,Vand an opaque photo-electric cathode, means mounting said cathodebetween said dynode and further electrode with the active surface ofsaid cathode facing said aperture in said dynode, said cathode having anarea small in relation to said dynode and said further electrode, toallow electrons released from said dynode to pass said cathode to saidfurther electrode, and means disposed between said window and saidapertured dynode to set up an electric field to repel electrons therebyto prevent substantially the passage of electrons from said cathodethrough the aperture in said dynode electrode.

3. An electron multiplier according to claim 2 wherein said meansdisposed between said window and said apertured dynode, is a translucentelectrically conductive coating arranged on said window.

4. An electron multiplier comprising an envelope having a translucentwindow, an apertured dynode electrode mounted in said envelope to facesaid window,

'an electron emitting layer on the surface of said dynode remote fromsaid Window, a further electrode spaced from said dynode, and an opaquephoto-electric cathode, means mounting said cathode Ibetween said dynodeand further electrode with the active surface of said cathode facingsaid aperture in said dynode, said cathode having an area small inrelation to said dynode and said further electrode, to allow electronsreleased from said dynode to pass said cathode to said furtherelectrode, electron focussing means to direct electrons from said dynodepassed said cathode onto said further electrode and means disposedbetween said window and said apertured dynode to set up an electricfield to repel electrons thereby to prevent substantially the passage ofelectrons from said cathode through the aperture in said dynodeelectrode.

5. An electron multiplier according to claim 4 wherein said meansdisposed between said window and said apertured dynode is a translucentelectrically conductive coating arranged on said window.

6. A multi-stage electron multiplier comprising an envelope having atranslucent window, an apertured dy- 1 node electrode mounted in saidenvelope to face said window, an electron emitting layer on the surfaceof said dynode remote from said window, at least one further dynodeelectrode spaced from said apertured dynode an electron collectingelectrode arranged to receive electrons from said further dynode and anopaque photo-electric cathode, means mounting said cathode between saidapertured dynode and said further dynode with the active surface of saidcathode having an area small in relation to the active surface area ofsaid apertured dynode so as to provide an open space surrounding saidcathode, to allow electrons released from said dynode to pass saidcathode to impinge on said further dynode and electrons released fromsaid further dynode to be collected by said collecting electrode.

Gorlich Aug. 17, 1937 Orthuber et al Dec. 31, 1940

